I rely on Pinguin Audio Meter standard version on Windows. I think they offer a 30-day demo, and it's not too expensive either at 245€. I really like the high-res spectrum analyzer and its stereo-meter. I use this thing constantly when mixing so it's well worth its price.
WaveLab has similar tools built into it that aren't quite as responsive but has an an RMS meter among other things.
I don't know of any freeware tools, sorry!